A new method for estimation of chain lengths of ribonucleic acids is under development. The method involves partial heat degradation of the RNA under study and measurements of the remaining and degraded molecules. From similar measurements on reference RNAs, the chain lengths may be deduced. In addition, computer simulation permits computation of the relation between chain length and electrophoretic mobility for use in electrophoretic analysis of polydisperse samples. Heterogenous nuclear RNA from HeLa cells has been studied by these techniques. Purification of RNA and DNA by means of conventional and electrophoretic techniques has been achieved, principally through the use of apparatus designed elsewhere. The purified material facilitates study of the properties of these molecules.